Biomechanics

The Classroom Ballista

By Steven Vogel, Duke University


Type of Resource Class activity
 
Topic Other 
Taxa Vertebrate
 
Organizational Level Organismal
 
Estimated time to do activity 5 minutes 
Background required/level Any 
Role of activity in your course I bring it in when I talk about the properties of materials--it brings home the relevance of Young's modulus, strain energy storage, and resilience. I return to it later, when talking about maximum output of humans, in particular the power and force we can get from our muscles.  
What students might learn from this course or activity Concepts: distinguishing among at least three properties of materials, the practical implications of the limitations of humans as engines, the use of biomechanically-based calculations to make assertions about the history of technology.  
Special tools, equipment or software needed No special material is needed, but (assuming one has access to a saber saw and electric drill at least) one should anticipate spending $10 or $20 on wood, rope, screws, etc. Allow, say, half a day for building and testing.  
Safety precautions, possible permissions necessary None.  
Miscellaneous advice - pitfalls to avoid Some adjusting may be needed to ensure that the projectile goes through the hole in the front frame. Don't try shooting water balloons until that aspect of performance is reliable.

I think the thing could be built with machine screws and wingnuts in a form that collapses for travel--one mainly needs to make the front frame removable. With negligible use of metal, it should not bother airline inspections especially.  
Frequently asked questions by students They ask about scaling the thing up; I suggest a Google search of "ballista" and "ballistae," with, of course, the usual skepticism about the trust one puts in unvetted sources.  
Evaluation Not specifically. It does, at the least, enliven a lecture, either for class or as a visitor elsewhere.  
Description A simple, tabletop model of this bit of ancient artillery illustrates the way collagen can provide elastic energy storage--as it does in running. It also raises biomechanical questions that students can address with appropriate calculations--the weight of tendon needed for a specified shot, the frequency with which two artillerymen could shoot projectiles, and the dimensions of the capstan-turning rig they must have used.  
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